High-pressure decorative laminates have been widely employed in the building industry as counter and table tops, bathroom and kitchen work surfaces, furniture, cabinets, wall paneling, partitions, and doors. Because they are durable and resistant to scratching and heat, high-pressure decorative laminates have been popular in the furniture industry, primarily as tops for furniture.
High-pressure decorative laminates are laminated articles comprising plural layers of resin impregnated paper sheets, consolidated or bonded together into a unitary structure under heat and pressure. Conventionally, the decorative or print layer is a sheet of high quality, purified alpha cellulose fibers, which may contain various fillers and/or pigments, impregnated with a thermosetting condensation resin, such as aminotriazine-aldehyde resins, for example, melamine-formaldehyde resins. An overlay sheet, transparent when cured, may be employed to protect the decorative or print layer and is also a sheet of alpha cellulose fibers, or the like, impregnated with an aminotriazine aldehyde resin.
The overlay and print sheets are bonded to a plurality of core or body sheets of fibrous cellulosic material, usually Kraft paper, most generally impregnated with a thermosetting phenol-formaldehyde resin. The major portion of the paper in a decorative laminate is composed of the core or body sheets rather than the print or overlay sheets. Typically seven to eleven core sheets are consolidated with only a single print and a single overlay sheet, to form a conventional 1/16 inch decorative laminate.
Although the core sheets are less expensive than the print or overlay sheets, it is apparent that the core sheets are a significant cost factor, because of their volume in a decorative laminate. It is also apparent that many of the properties of the paper-base decorative laminates are derived from the papers employed as well as the resins employed therein. The properties of the core stock paper and resin, then, will influence the properties of the end product decorative laminate. Such high-pressure decorative laminates are well known, and taught for example by Palazzolo et al., in U.S. Pat. No. 4,060,450, where the Kraft paper core sheets were made from hardwood and softwood cellulosic fibers containing up to a 15 percent lignin content.
A variety of problems are associated with high-pressure decorative laminates. The phenolic resin used to impregnate the core sheets is usually a mixture of phenol, aqueous aldehyde such as formaldehyde, and sodium hydroxide, the latter component of which is an alkaline condensation material tha catalyzes yet controls the reaction, and allows a high phenol:formaldehyde mole ratio, i.e., up to about 1:1.5. The use of acidic materials, such as strong organic or inorganic acids, as condensation catalysts could lead to explosive reactions. Recently, the cost of phenol has dramatically increased, since it is a by-product of the chemical processing of crude oil fractions. Additionally, in very high humidity or tropical climates, these types of laminates have a tendency to absorb water and to blister over a period of years, causing possible complete disintegration of the laminate core.
Read, in U.S. Pat. No. 3,551,405, attempted to solve phenol cost problems by substituting desulfonated lignin for from 15 percent to 65 percent of the phenol. Read pre-reacted lignosulfonates with caustic alkali, such as sodium hydroxide, in the presence of alcohol at 250.degree. C. and about 600 psi., and then acidified the product to produce sulfur free lignin precipitate solids. This sulfur free lignin was then added to a phenolic resin, consisting of an admixture of phenol, aqueous formaldehyde and sodium hydroxide. The mixture was then diluted with major amounts of alcohol, to provide a low viscosity impregnating resin for Kraft paper core sheets. After impregnation into eighteen sheet core lay-ups, drying to the "B" stage, and consolidation at 1,250 psi. and 150.degree. C. for 30 minutes, laminated cores were produced having weight increase values of from 3.1 wt. % to 27.6 wt. %, due to water absorption, presumably at 25.degree. C. for a 24 hour period.
The Read process, while dramatically reducing phenol costs, does not appear to solve long term blistering problems, involves complicated sulfonated lignin pre-treatment, and introduces a major quantity of alcohol diluent that could pose pollution problems during "B" stage drying. Additionally, unless the alcohol is somehow recovered during "B" stage drying or lamination, its use would considerably offset the phenol cost saving. There has been a long felt need for a simplified process of manufacturing humidity resistant, high-pressure decorative laminates, that would allow substantial phenol substitution, not add to pollution problems, and which would completely eliminate high humidity, long range blistering problems.